The present invention relates to a planar lens for use in the art of optical communication, optical data processing, etc., and a production method thereof.
To cope with the rise in capacity of communications or the rise in the amount of data to be processed in data processing, the utilization of parallelism of light has been noted as an extremely effective means. In order to utilize parallelism of light, it is desired that optical fibers, which are optical transmission means, be arranged in arrays, not to mention light source and optical detector. It is necessary that lenses, which are light coupling means, be arrayed as well.
A planar lens array having many fine lens formed in a transparent flat substrate has excellent characteristics of arrayed lens. Since such a planar lens array is prepared by photolithography, a fine lens having a diameter of not greater than 1 mm can be easily prepared. Further, these lenses can be arrayed with a high precision. Accordingly, these lenses can be easily aligned with the optic axis of other arrayed optical elements and thus are suitable for parallel optical transmission and data processing.
A representative method for producing a planar lens array is disclosed in JP-A-61-201639. A glass substrate is coated with a metallic titanium film as an ion diffusion preventive mask. The titanium film is then provided with a circular aperture for diffusion of ion using an ordinary photolithographic technique. The glass substrate thus masked is then dipped in a molten salt mixed with nitrate so that alkaline metal ions in the glass substrate are exchanged with components for increasing the refractive index of glass such as thallium (Tl) ion and silver (Ag) ion in the molten salt. In this manner, a distribution of refractive indices corresponding to the distribution of concentration of hemispherically dispersed ions is formed in the glass substrate. This distribution acts as a lens.
In addition, descriptions of graded refractive index lenses can be found in JP-A-61-26535 and JP-A-61-132541. More specifically, these documents disclose in their Examples that ion exchange took place between a glass plate in which P2O5, Na2O, K2O and Al2O3 are contained and a fused salt made of silver nitrate and potassium nitrate. Further, they disclose that the distribution curve patterns showing refractive index-distributions within the refractive index distribution region were upwardly convex in shape.
When Tl is used as a refractive index-increasing component, the resulting lens can be provided with a raised numerical aperture. However, since the rate of diffusion of Tl ion in the glass substrate is small, it takes much time to prepare a lens having a diameter of not smaller than 100 xcexcm.
Moreover, the arts disclosed in the above-cited documents JP-A-61-26535 and JP-A-61-132541 relate to treatment for exchanging ions between Ag ions and Na or/and K ions in phosphate glass.
On the other hand, when Ag is used as a refractive index-increasing component, a lens having a diameter of not smaller than 100 xcexcm can be easily prepared because the rate of diffusion of Ag ion in the glass substrate is high.
However, the phosphate glass containing about 48 weight % P2O5, about 20 weight % Na2O and about 9 weight % K2O is used as mother glass in the documents JP-A-61-26535 and JP-A-61-132541. In this glass, phosphoric acid acts as network former, and besides, alkalis are contained in high amounts. Therefore, such glass has a drawback of being poor in chemical resistance.
The invention has been made in order to solve such problems. Therefore, an aim of the invention is to provide a planar lens having a great numerical aperture acquired by diffusion of Ag as a refractive index-increasing component into a glass substrate containing Li and having excellent chemical resistance and a method of producing the aforesaid planar lens.
The invention is intended for a hemispherical planar lens in which a refractive index-increasing component is diffused in a substantially hemispherical form or a semicylindrical (lenticular) planar lens in which a refractive index-increasing component is diffused in a substantially semicylindrical form. These planar lenses are characterized by utilizing Ag as a refractive index-increasing component and further containing in a glass substrate at least Li as a component of alkali metal to undergo ion exchange for Ag.
In the case of using Ag as a refractive index-increasing component, long-range diffusion of Ag is controlled to a greater extent as a difference between the ionic radii of Ag and an element exchanged for Ag becomes bigger. As a result, the distribution curve of refractive index comes to have a convex form in the positive direction of the coordinate axis for the refractive index n.
However, the condition of Ag diffusion is also influenced by factors other than the ionic radii of ions participating in ion exchange. For instance, the state of chemical bonds in glass is one of such factors. Specifically, the stronger the interaction between Ag and elements forming a network structure of the glass, the more greatly the diffusion of Ag is retarded.
When a refractive index distribution curve of the present planar lens is rendered convex in the positive direction of the coordinate axis for the refractive index, the numerical aperture of the lens can be enlarged for the following reason.
More specifically, the refractive index at some diffusion length is greater than that in a xe2x80x9clinearxe2x80x9d distribution, provided that the refractive index distribution curve is convex in the positive direction of the coordinate axis for the refractive index n within the distribution region of refractive indices of a planar lens. In other words, the refraction of light at the diffusion length satisfying the above condition becomes great and the focal length of the lens can be shortened. As a result, the numerical aperture of the lens can be enlarged. Additionally, the present planar lens is large in numerical aperture and sufficiently practicable as far as the numerical apertures thereof are of the order of 0.1.
(1) A planar lens of a first embodiment of the present invention, in which a refractive index distribution is formed by diffusing a refractive index-increasing component into a flat glass substrate in a substantially hemispherical form or semicylinder form,
wherein the refractive index-increasing component is silver, and the flat glass substrate comprises Li as an alkali metal that is subject to an ion exchange with the silver
(2) The planar lens as described in the item (1), wherein, when taking a radial distance: r from the center of the hemisphere or the semicylinder and a refractive index: n at the radial distance r as coordinate axes orthogonal to each other, a distribution curve of the refractive index: n with respect to the radial distance: r is a convex curve in the positive direction of the coordinate axis for the refractive index: n.
(3) The planar lens as described in the item (1), wherein, when the distribution of refractive indices is expressed as a function: n(r) of a radial distance from the center of the hemisphere or the semicylinder: r and a refractive index: n, the second derivative: nxe2x80x3(r) of the function n(r) is negative or zero.
(4) The planar lens as described in the item (1), wherein the flat glass substrate comprises:
0 to 80 mol % SiO2;
0 to 80 mol % B2O3;
0 to 80 mol % P2O5;
3 to 45 mol % Li2O;
0 to 20 mol % Na2O;
0 to 30 molt MgO;
0 to 30 mol % CaO;
0 to 30 mol % SrO;
0 to 30 mol % BaO;
0 to 30 mol % ZrO2;
0 to 30 mol % Y2O3;
0 to 20 mol % La2O3;
0 to 40 mol % Al2O3; and
0 to 20 mol % Sm2O3,
wherein the sum of SiO2, B2O3 and P2O5 is 30 to 80 mol %, and the sum of Li2O and Na2O is 3 to 65 mol %.
(5) The planar lens as described in the item (1), wherein the flat glass substrate comprises:
30 to 80 mol % SiO2;
0 to 60 mol % B2O3;
3 to 45 mol % Li2O;
0 to 20 mol % Na2O;
0 to 30 mol % MgO;
0 to 30 mol % CaO;
0 to 30 mol % SrO;
0 to 30 mol % BaO;
0 to 30 mol % ZrO2;
0 to 30 mol % Y2O3;
0 to 20 mol % La2O3;
0 to 40 mol % Al2O3; and
0 to 20 mol % Sm2O3,
wherein the sum of SiO2 and B2O3 is 30 to 80 mol %, and the sum of Li2O and Na2O is 3 to 65 mol %.
(6) The planar lens as described in the item (5), wherein the flat glass substrate comprises B2O3 in an amount of 0 to 25 mol %;
(7) The planar lens as described in the item (1), wherein the flat glass substrate comprises:
30 to 80 mol % SiO2;
0 to 60 mol % B2O3;
3 to 45 mol % Li2O;
0 to 20 mol % Na2O;
0 to 30 mol % MgO;
0 to 30 mol % CaO;
0 to 30 mol % SrO;
0 to 30 mol % BaO;
0 to 30 mol % ZrO2;
0 to 30 mol % Y2O3;
0 to 20 mol % La2O3;
0 to 40 mol % Al2O3; and
0 to 20 mol % Sm2O3,
wherein the sum of SiO2 and B2O3 is 30 to 80 mol %, and the sum of Li2O and Na2O is 3 to 65 mol %.
(8) A method for producing a planar lens of a second embodiment of the present invention, comprising forming a refractive index distribution by diffusing a refractive index-creasing component into a flat glass substrate in a substantially hemispherical form or semicylinder form,
wherein the refractive index-increasing component is silver, and the flat glass substrate comprises Li as an alkali metal that is subject to an ion exchange with the silver.
(9) The method for producing a planar lens as described in the item (8), wherein, when taking a radial distance: r from the center of the hemisphere or the semicylinder and a refractive index: n at the radial distance r as coordinate axes orthogonal to each other, a distribution curve of the refractive index: n with respect to the radial distance: r is a convex curve in the positive direction of the coordinate axis for the refractive index: n.
(10) The method for producing a planar lens as described in the item (8), wherein, when the distribution of refractive indices is expressed as a function: n(r) of a radial distance from the center of the hemisphere or the semicylinder: r and a refractive index: n, the second derivative: nxe2x80x3(r) of the function n(r) is negative or zero.
(11) The method for producing a planar lens as described in the item (8), wherein the flat glass substrate comprises:
0 to 80 mol % SiO2;
0 to 80 mol % B2O3;
0 to 80 mol % P2O5;
3 to 45 mol % Li2O;
0 to 20 mol % Na2O;
0 to 30 molt MgO;
0 to 30 mol % CaO;
0 to 30 mol % SrO;
0 to 30 mol % BaO;
0 to 30 mol % ZrO2;
0 to 30 mol % Y2O3;
0 to 20 mol % La2O3;
0 to 40 mol % Al2O3; and
0 to 20 mol % Sm2O3,
wherein the sum of SiO2, B2O3 and P2O5 is 30 to 80 mol %, and the sum of Li2O and Na2O is 3 to 65 mol %.
(12) The method for producing a planar lens as described in the item (8), wherein the flat glass substrate comprises:
30 to 80 mol % SiO2;
0 to 60 mol % B2O3;
3 to 45 mol % Li2O;
0 to 20 mol % Na2O;
0 to 30 mol % MgO;
0 to 30 mol % CaO;
0 to 30 mol % SrO;
0 to 30 mol % BaO;
0 to 30 mol % ZrO2;
0 to 30 mol % Y2O3;
0 to 20 mol % La2O3;
0 to 40 mol % Al2O3; and
0 to 20 mol % Sm2O3,
wherein the sum of SiO2 and B2O3 is 30 to 80 mol %, and the sum of Li2O and Na2O is 3 to 65 mol %.
(13) The method for producing a planar lens as described in item (12), wherein the flat glass substrate comprises B2O3 an amount of 0 to 25 mol %;
(14) The method for producing a planar lens as described in the item (8), wherein the flat glass substrate comprises:
30 to 80 mol % SiO2;
0 to 60 mol % B2O3;
3 to 45 mol % Li2O;
0 to 20 mol % Na2O;
0 to 30 mol % MgO;
0 to 30 mol % CaO;
0 to 30 mol % SrO;
0 to 30 mol % BaO;
0 to 30 mol % ZrO2;
0 to 30 mol % Y2O3;
0 to 20 mol % La2O3;
0 to 40 mol % Al2O3; and
0 to 20 mol % Sm2O3,
wherein the sum of SiO2 and B2O3 is 30 to 80 mol %, and the sum of Li2O and Na2O is 3 to 65 mol %.
In the aforementioned ranges of concentration, the concentration of SiO2 and B2O3 more preferably falls within the following ranges. The use of SiO2 as an essential component with the addition of a proper amount of B2O3 makes it possible to obtain an effect of enhancing the chemical resistance of the glass substrate.
30xe2x89xa6SiO2xe2x89xa680; and
0xe2x89xa6B2O3xe2x89xa660
Even more preferably, the concentration of SiO2 and B2O3 falls within the following ranges. In this arrangement, the chemical resistance of the glass substrate can be further enhanced.
30xe2x89xa6SiO2xe2x89xa680; and
0xe2x89xa6B2O3xe2x89xa625
Alternatively, the concentration of SiO2 and B2O3 may fall within the following ranges, in which SiO2 is not an essential component and B2O3 is an essential component. When B2O3 is diffused as an essential component in the glass substrate in the following range of concentration, the wavefront aberration, which is one of optical properties of lens, of the resulting glass substrate can be reduced.
0xe2x89xa6SiO2xe2x89xa660; and
30xe2x89xa6B2O3xe2x89xa680
When the glass substrate containing at least Li in such a way undergoes ion exchange for Ag through circular or linear apertures of a mask, planar lenses having lens diameters of, e.g., 100 xcexcm or larger can be obtained with ease.
Further, when taking a radial distance r from the center of the hemisphere or the semicylinder and a refractive index n at the radial distance r as coordinate axes orthogonal to each other, the distribution curve of the refractive index n with respect to the distance r becomes convex in the positive direction of the coordinate axis for the refractive index n. As a result, planar lenses big in numerical aperture can be obtained.
Additionally, the inventions described in the items (4) and (11) include the cases of using phosphoric acid as a network former of glass. The present inventions contain Li as one of alkali metals to be exchanged for Ag. So the total amount of alkali metals required for producing lenses equal in performance can be reduced, and thereby the lenses obtained can be prevented from being extremely inferior in chemical resistance even when they use phosphoric acid as a network former of the glass.
By contrast, the phosphate glasses of the above-cited documents JP-A-61-26535 and JP-A-61-132541 contain Na and K as alkali metals to be exchanged for Ag. Therefore, the total amount of alkali metals required for attaining the desired lens performance is large, and thereby deterioration in chemical resistance is caused.
The preferred range of composition of the matrix glass is set forth below. The concentration of the net work former components of glass are 0xe2x89xa6SiO2 less than 80 (hereinafter, unit is mol-%), 0xe2x89xa6B2O3 less than 80 and 0xe2x89xa6P2O5xe2x89xa680, with the proviso that the sum of the concentration of these components (SiO2+B2O3+P2O5) is from not smaller than 30 to not greater than 80. The concentration of the alkaline metal oxides are 3xe2x89xa6Li2Oxe2x89xa645 and 0xe2x89xa6Na2Oxe2x89xa620, with the proviso that the sum of these components is from not smaller than 3 to not greater than 65. The concentration of the network-modifier oxides and other components are 0xe2x89xa6MgOxe2x89xa630, 0xe2x89xa6CaOxe2x89xa630, 0xe2x89xa6SrOxe2x89xa630, 0xe2x89xa6BaOxe2x89xa630, 0xe2x89xa6ZrO2xe2x89xa630, 0xe2x89xa6Y2O3xe2x89xa630, 0xe2x89xa6La2O3xe2x89xa620, 0xe2x89xa6Al2O3xe2x89xa640, and 0xe2x89xa6Sm2O3xe2x89xa620.
The range of concentration of various components of these glass compositions will be further described hereinafter. (Li2O)
In order to form an upwardly convex curve of distribution of refractive indices, it is essential that the alkaline ions to be exchanged with silver ion include Li ion. When the refractive index distribution curve or Ag concentration distribution curve is an upwardly convex, the numerical aperture (NA) of lens is raised. When the concentration of Li2O falls below 2%, n2/n1 ratio or a2/a1 ratio is reduced. On the contrary, when the concentration of Li2O exceeds 50%, devitrification occurs during cooling and solidification, making it impossible to form glass. Thus, it is necessary that the concentration of Li2O be from 3 to 45%, preferably from 6 to 40%, more preferably from 8 to 35%.
(Na2O)
The incorporation of Na2O makes it possible to accelerate the homogenization of glass and raise the numerical aperture. However, as the concentration of Na2O rises beyond 25%, n2/n1 ratio or a2/a1 ratio decreases to reduce the numerical aperture. Thus, it is necessary that the concentration of Na2O be not greater than 20%, preferably not greater than 15%, more preferably not greater than 10%.
However, when the total concentration of alkaline metal oxides to be exchanged with Ag ion falls below 3%, the resulting lens has a small numerical aperture and thus is not practical. On the contrary, when the total concentration of alkaline metal oxides to be exchanged with Ag ion exceeds 65%, the resulting glass shows a sudden deterioration of durability. Thus, it is necessary that the total concentration of alkaline metal oxides to be exchanged with Ag ion fall within the range of from 3 to 65%, preferably from 6 to 55%, more preferably from 8 to 45%.
(SiO2, B2O3 and P2O5)
SiO2, B2O3 and P2O5 are components constituting the net work former of glass. When the sum of concentration of the three components falls below 25%, the resulting glass exhibits deteriorated durability and stability. On the contrary, when the sum of concentration of the three components exceeds 85%, the resulting glass exhibits a raised melting temperature and a required amount of other constituents cannot be secured, making it impossible to obtain a lens having a great difference of refractive index. Thus, it is necessary that the sum of concentration of the three components fall within the range of from 30 to 80%, preferably from 35 to 75%, more preferably from 40 to 70%.
(SiO2)
SiO2 is a component constituting the skeleton of glass. When the concentration of this component exceeds 85%, the resulting glass exhibits a raised melting temperature and a required amount of other constituents cannot be secured, making it impossible to obtain a lens having a great difference of refractive index. Thus, it is necessary that the concentration of SiO2 fall within the range of not greater than 80%, preferably not greater than 60%.
(B2O3)
B2O3 is a component constituting the skeleton of glass. When the concentration of this component exceeds 85%, the resulting glass exhibits deteriorated durability and stability. Thus, the content of B2O3 is not greater than 80%, preferably not greater than 60%.
(P2O5)
P2O5 is a component constituting the skeleton of glass. When the concentration of this component exceeds 85%, the resulting glass exhibits deteriorated durability and stability. Thus, the content of B2O3 is not greater than 80%, preferably not greater than 60%.
(MgO)
MgO is an arbitrary component which acts as a network-modifying oxide in glass to accelerate the dissolution of glass and enhance the solubility thereof. The higher MgO content is, the better is the efficiency of performance thereof. However, when the content of MgO exceeds 35%, the viscosity of the resulting molten glass is lowered more than required, causing devitrification during cooling and solidification and hence making it impossible to form glass. Thus, it is necessary that the content of MgO fall within the range of not greater than 30%, preferably not greater than 22%, more preferably not greater than 10%.
(CaO)
CaO is an arbitrary component which enhances the solubility of batch. When the content of CaO exceeds 35%, the rate at which silver ion is exchanged is lowered. Thus, it is necessary that the content of CaO fall within the range of not greater than 30%, preferably not greater than 19%, more preferably not greater than 8%.
(SrO)
SrO is an arbitrary component which enhances the solubility of batch. When the content of SrO exceeds 35%, the rate at which silver ion is exchanged is lowered. Thus, it is necessary that the content of SrO fall within the range of not greater than 30%, preferably not greater than 21%, more preferably not greater than 12%.
(BaO)
BaO exerts an extremely great effect of changing the refractive index or dispersion of glass. When the content of BaO exceeds 35%, the rate at which silver ion is exchanged is lowered. Thus, it is necessary that the content of BaO fall within the range of not greater than 30%, preferably not greater than 20%, more preferably not greater than 10%.
(ZrO2)
ZrO2 exerts an extremely great effect of enhancing the weathering resistance of glass. When the content of ZrO2 exceeds 35%, ZrO2 can easily become insoluble. Thus, it is necessary that the content of ZrO2 fall within the range of not greater than 30%, preferably not greater than 21%, more preferably not greater than 12%.
(Y2O3)
Y2O3 exerts an extremely great effect of changing the refractive index or dispersion of glass. When the content of Y2O3 exceeds 35%, the rate at which silver ion is exchanged is lowered. Thus, it is necessary that the content of Y2O3 fall within the range of not greater than 30%, preferably not greater than 19%, more preferably not greater than 8%.
(La2O3)
La2O3 exerts an extremely great effect of changing the refractive index or dispersion of glass. When the content of La2O3 exceeds 25%, the rate at which silver ion is exchanged is lowered. Thus, it is necessary that the content of La2O3 fall within the range of not greater than 20%, preferably not greater than 14%, more preferably not greater than 8%.
(Al2O3)
Al2O3 exerts an extremely great effect of enhancing the weathering resistance of glass. When the content of Al2O3 exceeds 45%, Al2O3 can easily become insoluble. Thus, it is necessary that the content of Al2O3 fall within the range of not greater than 40%, preferably not greater than 32%, more preferably not greater than 25%.
(Sm2O3)
Sm2O3 exerts an extremely great effect of changing the refractive index or dispersion of glass. When the content of Sm2O3 exceeds 25%, the rate at which silver ion is exchanged is lowered. Thus, it is necessary that the content of Sm2O3 fall within the range of not greater than 20%, preferably not greater than 14%, more preferably not greater than 8%.
As glass components according to the invention there may be used one or more components selected from the group consisting of ZnO, K2O, TiO2, Ta2O5, Sc2O3, WO3, Pr2O3, Nd2O3, Eu2O3, Gd2O3, Tb2O3, Dy2O3, Ho2O3, Er2O3, Tm2O3, Yb2O3 and Lu2O3 in a total amount of not greater than 10% besides the aforementioned components. These components exert an effect of enlarging the vitrification range and enhancing the glass solubility. However, when these components are used in amounts greater than the above defined range, ion exchange can difficultly proceed smoothly.
Further, for the purpose of refining the glass during melting, components such as As2O3 and Sb2O3 may be diffused besides these components in a total amount of up to 2%.